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United States Patent |
6,060,950
|
Groe
|
May 9, 2000
|
Control of a variable gain amplifier with a delta sigma modulator D/A
converter
Abstract
Apparatus, and an associated method, for generating a gain control signal
for controlling operation of a variable gain amplifier. Gain control
circuitry includes a delta sigma modulator which exhibits a noise transfer
function for shifting noise components of a gain control signal formed
therefrom upwards in frequency. Undesired modulation of portions of a
received signal to be amplified by the amplifier, together with noise
components of the gain control signal, thereby do not interfere with
operation of apparatus in which the variable gain amplifier forms a
portion.
Inventors:
|
Groe; John B. (Poway, CA)
|
Assignee:
|
Nokia Mobile Phones Limited (Espoo, FI)
|
Appl. No.:
|
092464 |
Filed:
|
June 5, 1998 |
Current U.S. Class: |
330/279; 330/129 |
Intern'l Class: |
H03G 003/10 |
Field of Search: |
330/279,129
455/232.1,234.1,234.2,239.1,240.1,250.1
375/345
|
References Cited
U.S. Patent Documents
5083304 | Jan., 1992 | Cahill | 375/98.
|
5283536 | Feb., 1994 | Wheatley, III et al. | 330/279.
|
5722062 | Feb., 1998 | Nakanishi et al. | 455/247.
|
5737033 | Apr., 1998 | Masuda | 348/678.
|
Primary Examiner: Lee; Benny
Assistant Examiner: Choe; Henry
Attorney, Agent or Firm: Patel; Milan I.
Holland & Hart LLP
Claims
We claim:
1. Gain control apparatus for selectively amplifying an input signal, said
gain control apparatus comprising:
a variable gain amplifier having selectable gain characteristics and
coupled to receive electrical signals representative of the input signal,
said variable gain amplifier for amplifying the electrical signals with a
selected gain characteristic;
a delta sigma modulator coupled to said variable gain amplifier in a gain
control loop and exhibiting a signal transfer function and a noise
transfer function, the signal transfer function and the noise transfer
function exhibiting dissimilar characteristics said delta sigma modulator
for generating a gain control signal of values which, when applied to said
variable gain amplifier, are determinative of the selected gain
characteristic by which the electrical signals are amplified, said delta
sigma modulator for shaping a noise component of the gain control signal,
such that the noise component is of frequencies offset in frequency by a
frequency offset at least a selected amount relative to a fundamental
frequency of the noise component.
2. The gain control apparatus of claim 1 wherein the gain control signal
generated by said delta sigma modulator comprises an analog gain control
signal.
3. The gain control apparatus of claim 1 wherein said delta sigma modulator
comprises a first order delta sigma modulator.
4. The gain control apparatus of claim 1 wherein the signal transfer
function exhibits substantially flat signal transfer characteristics over
a range of frequencies corresponding to a selected channel upon which the
input signal is received.
5. The gain control apparatus of claim 1 wherein the noise transfer
function exhibits attenuation over a range of frequencies corresponding to
a selected receive channel upon which the input signal is received.
6. The gain control apparatus of claim 1 wherein said delta sigma modulator
generates a delta sigma-modulated signal and wherein said delta sigma
modulator further comprises a digital-to-analog converter coupled to
receive the delta sigma-modulated signal, said digital-to-analog converter
for generating the gain control signal.
7. The gain control apparatus of claim 1 further comprising a signal
processor coupled in the gain control loop together with said delta sigma
modulator, to receive signals representative of the electrical signals,
once amplified by said variable gain amplifier, said signal processor
operable at least to execute a power control algorithm to form a power
control signal, the power control signal applied, by way of the gain
control loop, to said delta sigma modulator, the values of the gain
control signal, at least in part, responsive to the power control signal.
8. The gain control apparatus of claim 7 wherein the variable gain
amplifier forms a portion of a receiver, the receiver comprising a digital
circuit portion and an analog circuit portion, wherein said signal
processor comprises a digital signal processor, wherein said digital
signal processor is embodied in the digital circuit portion of the
receiver and said variable gain amplifier is embodied in the analog
circuit portion of the receiver.
9. The gain controller apparatus of claim 8 further comprising a
digital-to-analog converter coupled to said delta sigma modulator and
wherein said delta sigma modulator is embodied in the digital circuit
portion of the receiver and said digital-to-analog converter is embodied
in the analog circuit portion.
10. The gain control apparatus of claim 9 wherein said delta sigma
modulator generates a differential, delta sigma-modulated signal and said
digital-to-analog converter is coupled to receive the differential, delta
sigma-modulated signal.
11. A method for generating a gain control signal for controlling levels of
gain of a variable gain amplifier, said method comprising the steps of:
determining responsive to signals previously received and amplified by the
variable gain amplifier, at least whether to adjust the levels of gain of
the variable gain amplifier by applying indications of signal levels of
the signals previously received and amplified by the variable gain
amplifier to a signal processor operable at least to execute a power
control algorithm and executing the power control algorithm, thereby to
generate indications of determinations whether to adjust the levels of
gain of the variable gain amplifier, the indications of determinations a
digital signal having a first resolution; and
providing indicators of determinations made during said step of determining
to a delta sigma modulator, the delta sigma modulator exhibiting a noise
transfer function and a signal transfer function, the noise transfer
function dissimilar with the signal transfer function;
generating the gain control signal at the delta sigma modulator, the gain
control signal of characteristics determined by the noise transfer
function and the signal transfer function such that noise components of
the gain control signal are located beyond a selected range of
frequencies.
12. The method of claim 11 wherein said step of generating the gain control
signal comprises forming a delta sigma-modulated signal at the delta sigma
modulator responsive to the indications provided thereto during said step
of providing the delta sigma-modulated signal having a second resolution,
and converting the delta sigma-modulated signal into analog form, the
delta sigma-modulated signal, when converted into the analog form, forming
the gain control signal.
13. The method of claim 12 further for controlling the levels of gain of
the variable gain amplifier, said method comprising the further step of:
applying the delta sigma-modulated signal, once converted into analog form
to form the gain control signal, to the variable gain amplifier, the noise
components of the gain control signal shifted in frequency such that
unwanted modulation of the noise components with the signals received by
the variable gain amplifier forming unwanted modulation components are
located at frequencies beyond a selected range of frequencies.
14. In a radio device having radio circuitry, an improvement of gain
control apparatus, said gain control apparatus comprising:
a variable gain amplifier having selectable gain characteristics and
coupled to receive electrical signals generated during operation of the
radio circuitry, said variable gain amplifier for amplifying the
electrical signals with a selected gain characteristic;
a delta sigma modulator coupled to said variable gain amplifier in a gain
control loop, said delta sigma modulator for generating a gain control
signal of values which, when applied to said variable gain amplifier, are
determinative of the selected gain characteristic by which the electrical
signals are amplified, said delta sigma modulator for shaping noise
components of the gain control signal such that the noise components are
of frequencies offset at least a selected amount relative to a fundamental
frequency of the noise components; and
said delta sigma modulator further comprising a digital-to-analog
controller and said delta sigma modulator generating a delta
sigma-modulated signal and said digital-to-analog controller for
converting the delta sigma-modulated signal into an analog signal, the
analog signal forming the gain control signal.
15. The gain control apparatus of claim 14 wherein the radio circuitry
comprises a radio receiver and wherein the electrical signals to which
said variable gain amplifier is coupled to receive comprises intermediate
frequency, receive signals.
16. Gain control apparatus for selectively amplifying an input signal, said
gain control apparatus comprising:
a variable gain amplifier having selectable gain characteristics and
coupled to receive electrical signals representative of the input signal,
said variable gain amplifier for amplifying the electrical signals with a
selected gain characteristic;
a delta sigma modulator coupled to said variable gain amplifier in a gain
control loop, said delta sigma modulator for generating a gain control
signal of values which, when applied to said variable gain amplifier, are
determinative of the selected gain characteristic by which the electrical
signals are amplified, said delta sigma modulator for shaping a noise
component of the gain control signal, such that the noise component is of
frequencies offset in frequency by a frequency offset at least a selected
amount relative to a fundamental frequency of the noise component; and
said delta sigma modulator further comprising a digital-to-analog converter
coupled to delta sigma modulator wherein said delta sigma modulator is
embodied in the digital circuit portion of the receiver and said
digital-to-analog converter is embodied in the analog circuit portion.
Description
The present invention relates generally to a gain control circuit, such as
a gain control circuit forming a portion of a radio telephone. More
particularly, the present invention relates to apparatus, and an
associated method, for generating a gain control signal for controlling
the gain of a variable gain amplifier.
Noise shaping is performed upon the noise component of the gain control
signal to shift the frequency of the noise component. Because of such
shifting, interference sometimes otherwise caused through undesired
interaction between an input signal applied to the variable gain amplifier
to be amplified thereat and the noise component of the gain control signal
is avoided.
A delta sigma modulator, or other noise shaper, is connected in a feedback
arrangement with the variable gain amplifier and is used in the formation
of the gain control signal. Because the delta sigma modulator exhibits a
signal transfer function and a noise transfer function which are
dissimilar, noise-shaping of the noise component by the modulator is
provided to shift the noise component. The delta sigma modulator is
coupled to a digital-to-analog converter to convert the gain control
signal into analog form. Use of the delta sigma modulator also permits
increase in the resolution of signals applied thereto, thereby to permit
fine, i.e. high-resolution, gain control of the variable gain amplifier by
the gain control signal, all without affecting the time response of the
circuit.
BACKGROUND OF THE INVENTION
A communication system is operable to communicate information between a
transmitting station and a receiving station by way of a communication
channel. A radio communication system is a communication system in which
the communication channel by which information is communicated between the
transmitting and receiving stations is formed upon a portion of the
electromagnetic spectrum. A cellular communication system is exemplary of
a multi-user, radio communication system.
Various cellular communication systems have been developed and implemented
throughout large geographical areas. Cellular communication systems have
been developed and implemented utilizing FDMA (frequency division multiple
access), TDMA (time division multiple access), CDMA (code division
multiple access), and various combinations of such communication
techniques.
Communication systems utilizing CDMA communication techniques
advantageously provide the possibility of increased communication capacity
levels within a given frequency bandwidth allocated to the communication
system. That is to say, CDMA communication techniques provide the
possibility to transmit a plurality of signals simultaneously over a
common bandwidth. Because of the simultaneous nature of communications in
a CDMA communication system, particular attention must be given to the
power levels of signals communicated on such shared bandwidth.
Gain control circuitry is utilized at a transmitting station to control the
signal levels of signals transmitted therefrom upon the communication
channel. And, the receiving station includes gain control circuitry for
modifying the gain of signals representative of receive signals received
at the receiving station. Gain control circuitry is utilized in
transmitting and receiving stations operable in other communication
systems, including the aforementioned cellular communication systems. And,
gain control circuitry is also used in other types of devices, used for
other purposes.
Receiver circuitry operable to process a signal transmitted upon a
communication channel sometimes receives not only a desired signal
component, but also interfering signal components. The interfering signal
components might be of greater signal levels than the desired signal
components. Signals transmitted during operation of a communication system
constructed according to, and in compliance with, the standards set forth
in the IS-98 specification pertaining to an analog/CDMA cellular
communication system, promulgated by the EIA/TIA, states the interfering
signal components to be as close as a 900 kHz frequency offset from the
center frequency of a desired signal and as large as 71 dBc above the
desired signal. This situation is commonly referred to as single tone
desensitization.
When such a receive signal is processed at a receiving station and applied
to a variable gain amplifier of variable gain circuitry, problems
sometimes occur. A variable gain amplifier, typically forms a portion of
the IF (intermediate frequency) portion of a receiver. The gain of the
variable gain amplifier is controlled by application of a gain control
signal thereto. If the gain control signal includes noise components, such
noise components, when applied to the variable gain amplifier together
with the receive signal, amplitude modulate interfering signal components
of the receive signal. The result of such undesired modulation includes
side bands which might obscure the desired portion, i.e., information
portion, of the receive signal.
While efforts have been made to overcome this problem, such efforts have
been constrained by the limited resolution permitted of existing
processing circuitry. Such processing circuitry sometimes forms a portion
of a gain control loop by which the gain of the variable gain amplifier is
controlled. That is to say, data words generated by digital processing
circuitry forming a portion of the gain control loop are of limited word
lengths. And, digital-to-analog converters, used to convert the data words
into analog form for application to the variable gain amplifier, are
limited to 8-10 bits. Another manner is a pulse density modulator (PDM)
which has a large switching component in its output.
A manner by which to prevent noise components of a gain control signal
applied to a variable gain amplifier of a gain control circuit from
interfering with operation of the variable gain amplifier would therefore
be advantageous.
It is in light of this background information related to gain control
circuitry that the significant improvements of the present invention have
evolved.
SUMMARY OF THE INVENTION
The present invention, accordingly, advantageously provides apparatus, and
an associated method, which generates a gain control signal for
application to a variable gain amplifier. Noise components which might
form portions of the gain control signal are suitably offset in frequency
so that any modulation of the noise components of the gain control signal
together with interfering signal components of a signal applied to the
variable gain amplifier to be amplified thereat shall not produce side
bands which might interfere with proper operation of the amplifier.
In one implementation, the gain control signal is formed in a gain control
circuit formed in a feedback connection with a variable gain amplifier.
The feedback loop includes a delta sigma modulator, coupled to receive as
inputs thereto, power control sequences generated by a power control
algorithm of a digital signal processor. The digital signal processor, in
turn, is coupled to receive, inter alia, indications of signal strength
levels of signals amplified by a variable gain amplifier.
The delta sigma modulator exhibits a signal transfer function which is
dissimilar to its noise transfer function. Because of such dissimilarity,
the delta sigma modulator is constructed in a manner to shape the noise
components of the sequences generated by the power control algorithm and
applied to the modulator to shift the noise components in frequency.
Sequences generated by the delta sigma modulator, i.e., the delta
sigma-modulated signal, are applied to a digital-to-analog converter. Once
converted into analog form, the analog signal forms the gain control
signal which is applied to the variable gain amplifier to control
amplification levels of the amplifier.
Because of the noise shifting by the delta sigma modulator of the noise
components, such components forming the analog, gain control signal
applied to the variable gain amplifier do not interfere with operation of
the variable gain amplifier. Undesired modulation of the noise components
of the gain control signal with interfering signal components of signals
applied to the amplifier to be amplified thereat do not generate side
bands which obscure the desired signal to be amplified by the amplifier.
In one implementation, the delta sigma modulator is constructed of CMOS
(complementary metal oxide semiconductor) components formed on-chip with,
or embodied in, a digital portion of a radio circuit. The
digital-to-analog converter and the variable gain amplifier are,
conversely, formed at an analog portion of the radio circuit.
In an exemplary implementation, an embodiment of the present invention
forms a portion of receiver circuitry of a cellular radio telephone
operable pursuant to the standards set forth in the IS-98 specification
promulgated by the EIA/TIA. In this implementation, the delta sigma
modulator, positioned in the feedback loop of a gain control circuit,
attenuates noise components within a range of frequencies at least
corresponding to the bandwidth of a receive channel upon which a receive
signal is received by the radio telephone and 900 kHz offsets at the upper
and lower ends of the receive-channel bandwidth. The delta sigma
modulator, together with a digital-to-analog converter, generates a gain
control signal for controlling amplification levels of a variable gain
amplifier of the radio telephone receiver circuitry.
The delta sigma modulator provides a flexible interface block which
controls operation of the variable gain amplifier. The modulator shapes
noise, and other spurious components out of the critical frequency band of
the receive channel, together with the frequency offsets. The delta sigma
modulator is operable over a wide, dynamic range and is operable without
delay of its time response.
In these and other aspects, therefore, gain control apparatus, and an
associated method, is provided for selectively amplifying an input signal.
A variable gain amplifier having a selectable gain characteristic is
coupled to receive electrical signals representative of the input signal.
The variable gain amplifier amplifies the electrical signals with a
selected gain characteristic. A gain characteristic controller is coupled
to the variable gain amplifier in a gain control loop. The gain
characteristic controller generates a gain control signal of values which,
when applied to the variable gain amplifier, are determinative of the
selected gain characteristic by which the electrical signals are
amplified. The gain characteristic controller shapes components of the
gain control signals such that the noise components are of frequencies
offset in frequency by a frequency offset at least a selected amount
relative to a fundamental frequency of the noise components.
A more complete appreciation of the present invention and the scope thereof
can be obtain from the accompanying drawings which are briefly summarized
below, the following detailed description of the presently-preferred
embodiments of the invention, and the appended claims.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 illustrates a functional block diagram which includes the gain
control circuitry of an embodiment of the present invention.
FIG. 2 illustrates a frequency representation of a receive signal,
exemplary of a receive signal received by the receiver circuitry shown in
FIG. 1.
FIGS. 3A and 3B illustrate frequency representations of an amplified signal
generated by a variable gain amplifier of the receiver circuitry shown in
FIG. 1 responsive to reception by the receiver circuitry of the receive
signal shown in FIG. 2.
FIG. 4 illustrates a functional block diagram of a delta sigma modulator
forming a portion of the gain control circuitry of an embodiment of the
present invention.
FIG. 5 illustrates an exemplary implementation of a portion of the delta
sigma modulator shown in FIG. 4.
FIG. 6 illustrates a representation of the frequency spectrum of the delta
sigma modulator shown in FIG. 4.
FIG. 7 illustrates a representation of the composite frequency spectrum of
a gain control signal generated during operation of an embodiment of the
present invention.
FIG. 8 illustrates a functional block diagram of a radio telephone which
includes an embodiment of the present invention.
FIG. 9 illustrates a method flow diagram listing the method steps of the
method of an embodiment of the present invention.
DETAILED DESCRIPTION
Referring first to FIG. 1, a receiver, shown generally at 10, is operable
to receive a receive signal 12. In the exemplary illustration, the
receiver 10 comprises a radio receiver, and the receive signal 12 is an
electromagnetic signal generated upon a communication channel defined upon
a portion of the electromagnetic spectrum. In other implementations, the
receiver 10 is alternately formed of another type of receiver operable to
receive a receive signal transmitted on another type of communication
channel.
The receive signal 12 is received by an antennae transducer 14 which
converts the electromagnetic waves forming the receive signal into the
electrical from on the line 16. The line 16 is here coupled to a low noise
amplifier 18 which is operable to amplify the electrical signal generated
upon the line 16. The amplifier 18 generates an amplified, electrical
signal on the line 22 which is applied to a first input of a down mixer
24. A down-mixing signal is applied to a second input of the down-mixer 24
by way of the line 26.
The down mixer 24 generates a first down-converted signal, here an
intermediate frequency signal, on the line 28. The intermediate frequency
signal generated on the line 28 is filtered by a filter 32. And, a
filtered, intermediate frequency signal, representative of the receive
signal 12, is generated on the line 34. The line 34 is coupled to an input
of a variable gain amplifier 36.
The variable gain amplifier 36 is operable to amplify the signal applied
thereto on the line 34 with a selected amount of gain and to generate an
amplified signal on the line 38. The level of gain by which the amplifier
36 amplifies the signal applied thereto on the line 34 is determined by a
gain control signal applied to the amplifier by way of the line 42.
The amplified signal generated on the line 38 is applied to a first input
of a second down mixer 44. A down mixing signal, generated on the line 46,
is applied to a second input of the down mixer 44. The down mixer 44 is
operable to generate a second down-mixed signal, here a base band signal
on the line 46. The base band signal generated on the line 46 is applied
to an A/D (analog-to-digital) converter 48. And, a digital base band
signal is generated on the line 52 and applied to a DSP (digital signal
processor) 54. The DSP 54 is operable to process the digital, base band
signal applied thereto. Such processing includes, inter alia, generation
of signals for application to an information sink 56, such as an acoustic
transducer.
The DSP 54 is also operable to execute a power control algorithm to
determine appropriate amplitude levels of amplified signals generated by
the amplifier 36. Sequences generated responsive to execution of the power
control algorithm are generated on the line 62 and are applied to gain
control circuitry 64. The gain control circuitry 64 functions as a
controller to control, through its operation, the gain of the amplifier
36. In manners which shall be described below, the gain control circuitry
64 is operable to generate a gain control signal on the line 42 to control
the gain of the variable gain amplifier 36. The gain control circuitry is
operable to generate a gain control signal in which noise component
portions of the gain control signal are shifted in frequency. The noise
component portions are shifted in manner to prevent amplitude modulation
by such noise components of a close-in interfering signal component of the
electrical signal representative of the receive signal applied on the line
34 to the amplifier 36, as described previously. The time response of the
gain control circuitry 64 permits a near-immediate generation of gain
control signal values responsive to input sequences provided thereto.
Further, the gain control circuitry 64 is operable over a wide, dynamic
range of operation.
FIG. 2 illustrates a frequency representation of an exemplary receive
signal 12 received at the receiver 10 during operation of an embodiment of
the present invention. In conventional fashion, the receiver 10 is
operable to receive and process the receive signal 12 transmitted on a
communication channel of a selected bandwidth. Here, the receive signal is
formed of a desired signal component 72 and interfering components 74. The
desired signal component is defined by a center frequency f.sub.c, and the
interfering components 74 is offset from the center frequency by at least
900 kHz. Electrical signal is generated on the line 16 and down-converted
signals generated, e.g., on the lines 28 and 34 are analogously
represented by the frequency representation shown in the Figure, with
appropriate down-conversion in frequency. The passband 75 of an IF
(intermediate frequency) filter is also shown in the Figure. As
illustrated, at increased frequencies, greater attenuation occurs. The
interfering component 74', e.g., is more greatly attentuated than is the
component 74. Filter 32 attenuates wide-spaced interferers but has only a
minor effect on close-in interferers.
FIG. 3A again illustrates the desired component of the receive signal 72,
here subsequent to amplification by the variable gain amplifier 36. A
modulated component 78 is also shown in the Figure. As illustrated, the
modulated component 78 overlaps with the desired signal component 36. The
modulated component 78 is representative of amplitude modulation of the
interfering component 74 (shown in FIG. 2) by noise components of the gain
control signal applied by way of the line 42 to the variable gain
amplifier 36 when the noise components are located in frequency close to
the frequency of the desired signal component 36. Operation of an
embodiment of the present invention shapes the noise components of the
gain control signal such that the modulated component 78 is shifted in
frequency in the direction indicated by the arrows 82. By shifting the
modulation component 78 away from the desired signal component 72,
corruption of the receive signal and desensitization of the receiver is
avoided.
FIG. 3B again illustrates the desired component of the receive signal 72,
here again subsequent to amplification by the variable gain amplifier 36.
A modulated component 84 is also shown in the Figure. Comparison of the
modulated component 84 with the modulated component 78 shown in FIG. 3A
indicates that the modulation by the noise component produced within the
delta sigma modulator D/A converter results in less desensitization.
FIG. 4 illustrates the gain control circuitry 64 of the gain control loop
68, shown in FIG. 1. The gain control circuitry 64 is here shown to
include a delta sigma modulator 86 and a digital-to-analog converter 88.
The delta sigma modulator 86 is coupled to receive the sequences generated
by the DSP 54 (shown in FIG. 1) on the line 62 and to generate delta
sigma-modulated signals on the line 92 for application to the
digital-to-analog converter 88. Analog signals generated by the converter
88 are generated on the line 94, filtered by a filter 96. The filtered
signal generated by the filter 96 form the gain control signals generated
on the line 42 for application to the amplifier 36 (shown in FIG. 1).
The delta sigma modulator 86 is functionally represented, here as a first
order device. The modulator 86 alternately is formed of a higher-order
device. The modulator 86 is here shown to include a quantizer 102, a
filter 104 having a gain B coupled in a feedback arrangement between an
output side of the quantizer 102 and to a negative input of a summing
device 106 by way of the line 107. The line 62 is coupled to a positive
input of the summing device 106. Values summed at the summing device 106
are generated on the line 108 which is coupled to a filter element 112.
The filter element 112 filters the sequences of values received thereat
and generates a filtered signal on the line 114 which is coupled to the
quantizer 102. The filter element 112 and the quantizer 102 together
define a gain A, here represented by the block 116, shown in dash. A line
118, also shown in dash, is representative of the application of noise and
spurious signals whose frequency characteristics are shaped during
operation of the modulator 86.
FIG. 5 illustrates an exemplary implementation of the filter 104 connected
in the feedback path of the modulator 86. The filter 104 here forms a FIR
(finite impulse response) filter having a series of delay elements 119 and
gain elements 120. The line 92 forms an input to the filter 104. The
values obtained from the separate paths of the filter are summed together
by a summer 121, and the summed result is generated on the output line
107.
The delta sigma modulator 86 is a differential pulse code modulator that
decorrelates a signal prior to its quantization. The correlation between
adjacent samples received by, and operated upon, by the modulator
increases as a square of the sampling frequency of such sequences.
Correspondingly, the variances of the decorrelated signal decreases as the
sampling frequency increases. Operation of the quantizer 102 is simplified
as a result of these principles.
The modulator 86 has associated therewith an SNR (signal-to-noise ratio).
The SNR performance of the modulator is affected by shaping the
quantization noise which is white noise. The signal is sampled at a
sampling rate significantly greater than the Nyquist rate, and the noise
is filtered by the filter element 112 prior to quantization. The modulator
86 is able to discriminate between signal components and noise components,
and the signal transfer function, STF, and the noise transfer function,
NTF, of the modulator 86 are distinct from one another.
As the gains of the elements 104 and 116 are represented by B(s) and A(s),
the STF and the NTF are represented as follows:
##EQU1##
The output of the modulator 86 thereby includes two components. A first
component is formed the input sequence provided on the line 62 and
modified by the STF of the modulator. And, a second component is formed of
a noise component, represented in FIG. 4 by the noise added by way of the
line 118 and modified by the NTF of the modulator. Through appropriate
selection of the values of A and B, the signal noise transfer functions
can be caused to exhibit desired characteristics. Namely, the STF is
caused to be of "flat" characteristics at low frequencies. And, the NTF is
caused to exhibit a high-pass response, attenuating quantization noise at
low frequencies and amplifying the noise at high frequencies. Thereby, the
NTF of the modulator provides noise shaping properties to the modulator to
describe its advantageous performance in a gain control loop.
The delta sigma modulator 86 is realized in digital form to receive N-bit
digital words on the input lines 62. Such words are combined with the
digitally-filtered version of the word generated by the quantizer 102.
Sequences summed by the summer 106 are acted upon by the filter 112, which
together with the quantizer 102 exhibits a gain of A. The quantizer 102 is
operable to truncate the sequences applied thereto, and to pass the most
significant bit thereof. The digital filters 104 and 112, summer 106, and
quantizer 102 all operate at a high sampling frequency. The delta sigma
modulator 86 is advantageously implemented with CMOS (complementary metal
oxide semiconductor) technology.
The converter 88 is here formed of a two-level D/A (digital-to-analog)
converter. And, the analog filter 96 is of filter characteristics to
attenuate high frequency noise amplified by the modulator 86. The
resultant gain control signal generated on the line 42 is an analog signal
representing the N-bit digital word applied to the modulator 86 on the
line 62.
FIG. 6 illustrates the frequency spectrum, shown at 92, of the delta sigma
modulator. The spectrum shows graphically the passband of the modulator,
through appropriate selection of the STF. At higher frequencies to which
noise components are shifted, the spectrum no longer attenuates such
components. But, the noise components are shifted high enough in frequency
not to interfere with circuit operation.
FIG. 7 illustrates the frequency response, shown at 42, of the delta sigma
modulator 86. The filter response 42 is determined by the sampling
frequency and the delay of the filters forming the modulator. The sampling
frequency is selected to be high to decrease the variance between
decorrelated signals and to simplify the quantizer 102 of the modulator.
The digital filter delay of the filters 104 and 112 is related to the
sampling frequency and the maximum tap delays of the FIR filter
implementations thereof. The tap delay is small so as not to affect the
time response of the modulator. And, the filter 96 is constructed to
reduce high frequency noise. The delay of the filter 96 is small compared
to a simple reconstructive filter, which is tied to data rates. The
response of the delta sigma modulator 86, in contrast, is independent of
the data rate. The digital-to-analog converter 86 and filter 96 can also
be realized by a delta sigma demodulator.
The converter 88 and the filter 96, being analog elements, are
advantageously integrated with the variable gain amplifier 36 (shown in
FIG. 1) using an analog process. The line 92 interconnecting the modulator
86 and the converter 88 is, in one embodiment, operable to provide a
single-ended signal. In another implementation, the line 92 includes
separate portions, and differential signals are applied to the converter
88.
FIG. 8 illustrates a cellular radio telephone, shown generally at 132,
which includes the gain control circuitry of an embodiment of the present
invention. The radio telephone includes a receiver portion to process
receive signals received at the antennae transducer 134. The transducer
134 converts electromagnetic signals into electrical signals which are
filtered by a filter portion 136 of a duplexer filter 138. Filtered
signals are applied to down-conversion circuitry 142, and down-converted
signals are applied to a variable gain amplifier 144. The amplifier 144 is
coupled to receive a gain control signal on the line 146, generated by
gain control circuitry 148.
Amplified signals generated by the amplifier 144 provided to second
down-conversion circuitry 152 whereat base band signals are formed and
provided to the DSP (digital signal processor) 154. The DSP 154 generates
the signals which are applied to an information sink 156. And, the DSP 154
is operable to execute a power control algorithm and to generate power
control sequences on the line 158 for application to the gain control
circuitry 148. The gain control circuitry 148 is analogous to the
circuitry 64 shown in FIG. 1. As connected, a gain control loop, indicated
by the arrow 162, is formed, thereby to control the levels of gain of the
variable gain amplifier 144.
The radio telephone 132 further includes a transmitter portion to transmit
signals generated by an information source 172. Signals generated by the
information source are provided to a modulator 174 to form modulated
signals according to an appropriate modulation technique. Modulated
signals generated by the modulator 174 are provided to a variable gain
amplifier 178. The variable gain amplifier 178 is also coupled to receive
a gain control signal generated on the line 182 by gain control circuitry
184.
Amplified signals generated by the amplifier 178 are provided to
up-conversion circuitry 186 which is filtered by filter circuitry 188 of
the duplexer 138 and provided to the antennae transducer 134. The
up-converted signals generated by the up-converter 186 are also provided
to an analog-to-digital converter 192. And, digitized signals are provided
to the gain control circuitry 184. The gain control circuitry 184
corresponds to the gain control circuitry 64 shown in FIG. 1. As
connected, a feedback loop, represented by the arrow 194 is formed.
FIG. 9 illustrates a method, shown generally at 202, of an embodiment of
the present invention. The method 202 provides a manner by which to gain a
generate control signal for controlling levels of gain of a variable gain
amplifier.
First, and as indicated by the block 204, a determination is made whether
to adjust the levels of gain of the variable gain amplifier responsive to
signals previously received and amplified by the variable gain amplifier.
Then, and as indicated by the block 206, indications of the determinations
made at the block 204 are provided to a gain characteristic controller.
The gain characteristic controller exhibits a noise transfer function and
a signal transfer function wherein the noise transfer function is
dissimilar with the signal transfer function.
Then, and as indicated by the block 208, a gain control signal is generated
at the gain characteristic controller. The gain control signal is a
characteristic determined by the noise transfer function and the signal
transfer function such that noise components of the gain control signal
are located beyond a selected range of frequencies.
Operation of an embodiment of the present invention thereby provides a
manner by which to control amplification levels of a variable gain
amplifier. Circuitry used to form the gain control signal provides noise
shaping properties for shaping noise components of the gain control signal
and is operable over a wide dynamic range.
The previous descriptions are of preferred examples for implementing the
invention, and the scope of the invention should not necessarily be
limited by this description. The scope of the present invention is defined
by the following claims.
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